[llvm] TargetInstrInfo: make getOperandLatency return optional (NFC) (PR #73769)

[Francesco Petrogalli via llvm-commits]

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@@ -173,11 +173,14 @@ unsigned TargetSchedModel::computeOperandLatency(
   const MachineInstr *DefMI, unsigned DefOperIdx,
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fpetrogalli wrote:

You may want to update the comment describing this interface, explaining why we do not need `std::optional` here.

https://github.com/llvm/llvm-project/pull/73769